Longitudinal study of posterior subcapsular opacities using the National Eye Institute computer planimetry system

BACKGROUND--The National Eye Institute (NEI) computer planimetry system has proved to be helpful in cross sectional studies by providing clinically useful area measurements of posterior subcapsular cataracts (PSC) and other opacities from retroillumination photographs. In this study, we evaluated the worth of this system in detecting PSC area changes over time. METHODS--Using the Neitz-Kawara camera, retroillumination photographs of the PSCs of 51 eyes were obtained every 6 months for an average of 25 months. The PSCs were outlined in a masked fashion on plastic overlays. Their tracings were then digitised on to a computer using a scanner. PSC area was determined using a special software program. For each eye, the rate of PSC area change was estimated by the slope of the regression line fitted to the follow up measurements. Cataract progression was classified as significant if the slope exceeded a critical value. RESULTS--These showed that 14 of the 51 eyes had PSC progression, while 37 did not change. CONCLUSION--This study suggests the system is useful for longitudinally monitoring PSC area changes from retroillumination photographs. This system would serve in natural history studies of PSCs and in clinical trials of anticataract drugs. The slope based test can play a part in longitudinal studies with irregular time intervals and variable number of visits.